U.S. patent number 10,056,611 [Application Number 15/371,614] was granted by the patent office on 2018-08-21 for method for testing cycle life of positive electrode active material for secondary battery.
This patent grant is currently assigned to LG Chem, Ltd.. The grantee listed for this patent is LG Chem, Ltd.. Invention is credited to Wang Mo Jung, Ji Hye Kim, Bo Ram Lee, Sun Sik Shin.
United States Patent |
10,056,611 |
Lee , et al. |
August 21, 2018 |
Method for testing cycle life of positive electrode active material
for secondary battery
Abstract
The present disclosure provides a method for testing a cycle
life of a positive electrode active material for a secondary
battery capable of predicting and assessing a cycle life of a
positive electrode active material with high accuracy and excellent
reliability in a short period of time using a simple method.
Inventors: |
Lee; Bo Ram (Daejeon,
KR), Kim; Ji Hye (Daejeon, KR), Shin; Sun
Sik (Daejeon, KR), Jung; Wang Mo (Daejeon,
KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
LG Chem, Ltd. |
Seoul |
N/A |
KR |
|
|
Assignee: |
LG Chem, Ltd.
(KR)
|
Family
ID: |
59066225 |
Appl.
No.: |
15/371,614 |
Filed: |
December 7, 2016 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170176543 A1 |
Jun 22, 2017 |
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Foreign Application Priority Data
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Dec 18, 2015 [KR] |
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10-2015-0181741 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R
31/388 (20190101); H01M 4/485 (20130101); H01M
10/4285 (20130101); G01R 31/3865 (20190101); H01M
10/052 (20130101); Y02E 60/10 (20130101); H01M
2004/028 (20130101); H01M 4/131 (20130101) |
Current International
Class: |
G01N
27/27 (20060101); G01R 31/36 (20060101); H01M
10/052 (20100101); H01M 10/42 (20060101); H01M
4/485 (20100101); H01M 4/02 (20060101) |
Field of
Search: |
;324/426 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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H117985 |
|
Jan 1999 |
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JP |
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2006153663 |
|
Jun 2006 |
|
JP |
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20150063254 |
|
Jun 2015 |
|
KR |
|
Primary Examiner: Aurora; Reena
Attorney, Agent or Firm: Lerner, David, Littenberg, Krumholz
& Mentlik, LLP
Claims
What is claimed is:
1. A method for testing a cycle life of a positive electrode active
material for a secondary battery comprising: (a) preparing a
half-cell having a positive electrode and a negative electrode,
wherein the positive electrode includes a positive electrode active
material and the negative electrode includes lithium; (b) charging
the half-cell to 4.4 V to 4.5 V under a constant current condition,
maintaining the half-cell for 5 hours to 10 hours in a voltage
range of 4.4 V to 4.5 V at 50.degree. C. to 80.degree. C., and
discharging the half-cell to 3.0 V under a constant voltage
condition to complete a charge/discharge cycle, and measuring
discharge capacity of the charge/discharge cycle; (c) repeating
step (b) for 5 to 10 charge/discharge cycles; and (d) calculating a
discharge capacity retention rate at each charge/discharge cycle
with respect to discharge capacity at the first charge/discharge
cycle using the measured discharge capacity at each
charge/discharge cycle.
2. The method for testing a cycle life of a positive electrode
active material for a secondary battery of claim 1, wherein the
charging step in each charge/discharge cycle is carried out under a
constant current condition of 0.2 C to 0.5 C.
3. The method for testing a cycle life of a positive electrode
active material for a secondary battery of claim 1, wherein the
maintaining step in each charge/discharge cycle is carried out for
8 hours to 10 hours in a voltage range of 4.45 V to 4.5 V at
60.degree. C. to 70.degree. C.
4. The method for testing a cycle life of a positive electrode
active material for a secondary battery of claim 1, wherein the
positive electrode active material includes a lithium composite
metal oxide having layered structure.
5. The method for testing a cycle life of a positive electrode
active material for a secondary battery of claim 4, wherein the
lithium composite metal oxide is doped with any one, or two or more
elements selected from the group consisting of Al, Cu, Fe, V, Cr,
Ti, Zr, Zn, Ta, Nb, Mg, B, W and Mo.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
The present application claims priority to and the benefits of
Korean Patent Application No. 10-2015-0181741 filed with the Korean
Intellectual Property Office on Dec. 18, 2015, the entire contents
of which are incorporated herein by reference.
TECHNICAL FIELD
The present disclosure relates to a method for testing a cycle life
of a positive electrode active material for a secondary battery
capable of predicting and assessing a cycle life of a positive
electrode active material with high accuracy and excellent
reliability in a short period of time.
DESCRIPTION OF THE RELATED ART
With increases in technology developments and demands for mobile
devices, demands for secondary batteries as an energy source have
rapidly increased. Among such secondary batteries, lithium
secondary batteries having high energy density and voltage, a long
cycle life and a low self-discharge rate have been commercialized
and widely used.
However, in lithium secondary batteries, active materials are
degenerated due to moisture inside the battery, or electrolyte
decomposition occurring during charge and discharge of the battery,
or the like. In addition, with an increase in the internal
resistance of the battery, a problem such as a rapid cycle life
decrease occurs as charge and discharge are repeated. Particularly,
such a problem is more serious at high temperatures.
Testing a cycle life of a battery cell is generally carried out
after actually manufacturing the cell. As one example, after
manufacturing a secondary battery cell, internal resistance of the
cell is corrected by temperature and measured, and the cycle life
of the battery is determined by comparing internal resistance
corrected at the measured temperature and internal resistance data
at a temperature set in advance. Another method is a method of
determining a life cycle of a secondary battery used through
repeating charge and discharge, and internal resistance in fixed
battery capacity is measured when charged. By calculating the
change in the internal resistance with the passage of time, the
cycle life of the secondary battery is determined. As another
method, there is a method of testing a cycle life of a battery by
repeating a significant number of charge and discharge such as 300
cycles, 500 cycles or 1,000 cycles for a battery cell depending on
the type of charge and discharge called constant current constant
voltage (CC-CV).
As described above, existing cycle life tests for battery cells are
carried out after manufacturing a cell of a secondary battery.
However, when manufacturing a cell without clearly identifying
positive electrode active material properties, target results are
not obtained sometimes after carrying out the cycle life test, and
the cell needs to be manufactured over again. In addition, long
time of several months is taken for tests, and it is difficult to
meet deadlines for new product development or delivery
inspection.
Accordingly, when a cycle life property of such a cell is capable
of being predicted in advance in a step of testing a positive
electrode active material even without manufacturing an actual
cell, efficient choice of a positive electrode active material
becomes possible, and as a result, cost savings may be accomplished
since the amount of cell manufacturing for actual tests is
reduced.
DISCLOSURE OF THE INVENTION
Technical Problem
The present disclosure is to provide a method for testing a cycle
life of a positive electrode active material for a secondary
battery capable of predicting and assessing a cycle life of a
positive electrode active material with high accuracy and excellent
reliability in a short period of time.
Technical Solution
The present disclosure has been made in view of the above, and one
embodiment of the present disclosure provides a method for testing
a cycle life of a positive electrode active material for a
secondary battery including preparing a positive electrode
including a positive electrode active material for testing cycle
life properties; preparing a half-cell using the positive electrode
and a lithium negative electrode; repeating 5 cycles to 10 cycles
with a charge and discharge process of charging the half-cell to
4.4 V to 4.5 V under a constant current condition, maintaining for
5 hours to 10 hours in a voltage range of 4.4 V to 4.5 V at
50.degree. C. to 80.degree. C., and discharging to 3.0 V under a
constant voltage condition as 1 cycle, and measuring discharge
capacity at each cycle; and calculating a discharge capacity
retention rate at each cycle with respect to discharge capacity at
the first cycle using the measured discharge capacity.
Other specifics of embodiments of the present disclosure are
included in the detailed descriptions provided below.
Advantageous Effects
A method for testing a cycle life of a positive electrode active
material for a secondary battery according to the present
disclosure is capable of predicting and assessing a cycle life of a
positive electrode active material with high accuracy and excellent
reliability in a short period of time. Accordingly, cycle life
properties of a cell can be readily predicted in a step of testing
the positive electrode active material without manufacturing and
directly testing the cell, which enables efficient choice of
positive electrode active materials, and is also economically
advantageous since cell manufacturing for actual tests is
reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings accompanied in the present specification
illustrate preferred embodiments of the present disclosure, and
further enlighten technological ideas of the present disclosure
together with the disclosure of the invention described above, and
therefore, the present disclosure is not to be interpreted to be
limited to the descriptions provided in the drawings.
FIG. 1 shows results of testing a cycle life property of positive
electrode active materials according to Example 1 to Example 3.
FIG. 2 shows results of testing a cycle life property of battery
cells according to Comparative Example 1 to Comparative Example
3.
MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present disclosure will be described in more
detail in order to illuminate the present disclosure.
Terms or words used in the present specification and the claims are
not to be interpreted limitedly to common or dictionary meanings,
and shall be interpreted as meanings and concepts corresponding to
technological ideas of the present disclosure based on a principle
in which the inventors may suitably define the concepts of terms in
order to describe the invention in the best possible way.
A method for testing a cycle life of a positive electrode active
material for a secondary battery according to one embodiment of the
present disclosure includes,
preparing a positive electrode including a positive electrode
active material for testing a cycle life property (Step 1);
preparing a half-cell using the positive electrode and a lithium
negative electrode (Step 2);
repeating 5 cycles to 10 cycles with a charge and discharge process
of charging the half-cell to 4.4 V to 4.5 V under a constant
current condition, maintaining for 5 hours to 10 hours in a voltage
range of 4.4 V to 4.5 V at 50.degree. C. to 80.degree. C., and
discharging to 3.0 V under a constant voltage condition as 1 cycle,
and measuring discharge capacity at each cycle (Step 3); and
calculating a discharge capacity retention rate at each cycle with
respect to discharge capacity at the first cycle using the measured
discharge capacity (Step 4).
Hereinafter, each step will be examined. First, Step 1 in the
method for testing a cycle life of a positive electrode active
material for a secondary battery according to one embodiment of the
present disclosure is a step of preparing a positive electrode
including a positive electrode active material for testing a cycle
life property.
The positive electrode may be prepared using common positive
electrode preparation methods except that an active material
subject to a cycle life test is used as a positive electrode active
material.
Specifically, the positive electrode may be prepared by coating a
composition for forming a positive electrode active material
prepared through dissolving or dispersing a positive electrode
active material subject to a cycle life test, and selectively, a
binder and a conductor in a solvent on a positive electrode current
collector, and then drying and rolling the result.
The positive electrode active material is a compound capable of
reversible lithium intercalation and deintercalation (lithiated
intercalation compound), and specifically, may include materials
having a hexagonal layered rock salt structure (specific examples
thereof may include LiCoO.sub.2,
LiCo.sub.1/3Mn.sub.1/3Ni.sub.1/3O.sub.2 or LiNiO.sub.2), materials
having an olivine structure (specific examples thereof may include
LiFePO.sub.4), spinel materials having a cubic structure (specific
examples thereof may include LiMn.sub.2O.sub.4), and in additions
to these, vanadium oxides such as V.sub.2O.sub.5, and chalcogene
compounds such as TiS or MoS.
More specifically, the positive electrode active material may be a
lithium composite metal oxide including lithium and metals such as
cobalt, manganese, nickel or aluminum. Specific examples of the
lithium composite metal oxide may include lithium-manganese-based
oxides (for example, LiMnO.sub.2, LiMn.sub.2O and the like),
lithium-cobalt-based oxides (for example, LiCoO.sub.2 and the
like), lithium-nickel-based oxides (for example, LiNiO.sub.2 and
the like), lithium-nickel-manganese-based oxides (for example,
LiNi.sub.1-Y1Mn.sub.Y1O.sub.2 (herein, 0<Y1<1),
LiMn.sub.2-z1Ni.sub.z1O.sub.4 (herein, 0<Z1<2) and the like),
lithium-nickel-cobalt-based oxides (for example,
LiNi.sub.1-Y2Co.sub.Y2O.sub.2 (herein, 0<Y2<1) and the like),
lithium-manganese-cobalt-based oxides (for example,
LiCo.sub.1-Y3Mn.sub.Y3O.sub.2 (herein, 0<Y3<1),
LiMn.sub.2-z2Co.sub.z2O.sub.4 (herein, 0<Z2<2) and the like),
lithium-nickel-manganese-cobalt-based oxides (for example,
Li(Ni.sub.P1Co.sub.Q1Mn.sub.R1)O.sub.2 (herein, 0<P1<1,
0<Q1<1, 0<R1<1, P1+Q1+R1=1) or
Li(Ni.sub.P2Co.sub.Q2Mn.sub.R2)O.sub.4 (herein, 0<P2<2,
0<Q2<2, 0<R2<2, P2+Q2+R2=2) and the like), or
lithium-nickel-cobalt-manganese-metal (M) oxides (for example,
Li(Ni.sub.P3Co.sub.Q3Mn.sub.R3M.sub.S1)O.sub.2 (herein, M is
selected from the group consisting of Al, Cu, Fe, V, Cr, Ti, Zr,
Zn, Ta, Nb, Mg, B, W and Mo, and P3, Q3, R3 and S1 are each an
atomic fraction of independent elements, and 0<P3<1,
0<Q3<1, 0<R3<1, 0<S1<1, P3+Q3+R3+S1=1) and the
like) and the like, and among these, any one, or two or more
compounds may be used.
In addition, in the lithium composite metal oxide, at least one of
metal elements other than lithium may be doped with any one, or two
or more elements selected from the group consisting of Al, Cu, Fe,
V, Cr, Ti, Zr, Zn, Ta, Nb, Mg, B, W and Mo. When the lithium
composite metal oxide is further doped with the above-mentioned
metal elements as described above, structural stability of the
positive electrode active material is improved. As a result,
battery output properties may be enhanced. Herein, the content of
the doping elements included in the lithium composite metal oxide
may be properly controlled in a range that does not decline
properties of the positive electrode active material. Specifically,
the content may be 1 atomic % or less, and more specifically, 0.02
atomic % or less.
More specifically, when considering that the method for testing a
cycle life of a positive electrode active material according to the
present disclosure is carried out under a severe condition of high
temperature and high voltage, cycle life property test results with
more accuracy and high reliability may be obtained without concern
for degeneration of the positive electrode active material itself.
Accordingly, the positive electrode active material may be a
layer-structured lithium composite metal oxide that is not doped or
is doped with any one, or two or more elements selected from the
group consisting of Al, Cu, Fe, V, Cr, Ti, Zr, Zn, Ta, Nb, Mg, B, W
and Mo. More specifically, the positive electrode active material
is a layer-structured lithium cobalt oxide that is not doped or is
doped with the above-mentioned doping elements.
Specifically, the lithium composite metal oxide may include a
compound of the following Chemical Formula 1.
Li.sub.1+aNi.sub.xCo.sub.yMn.sub.zM.sub.wO.sub.2 [Chemical Formula
1]
(In Chemical Formula 1, M may include any one, or two or more
elements selected from the group consisting of Al, Cu, Fe, V, Cr,
Ti, Zr, Zn, Ta, Nb, Mg, B, W and Mo, and a, x, y, z and w are each
independently an atomic fraction of the corresponding elements and
may be -0.5.ltoreq.a.ltoreq.0.5, 0.ltoreq.x.ltoreq.1,
0<y.ltoreq.1, 0.ltoreq.z.ltoreq.1, 0.ltoreq.w.ltoreq.1 and
0<x+y+z.ltoreq.1)
In addition, at least one or more of a conductor and a binder may
be selectively further used when preparing the composition for
forming a positive electrode active material layer.
The conductor is used for providing conductivity to the electrode,
and in the battery, is not particularly limited as long as it has
electron conductivity without inducing chemical changes. Specific
examples thereof may include graphite such as natural graphite or
artificial graphite; carbon-based materials such as carbon black,
acetylene black, Ketjen black, channel black, furnace black, lamp
black, thermal black and carbon fiber; metal powder such as copper,
nickel, aluminum and silver, or metal fiber; conductive whiskers
such as zinc oxide and potassium titanate; conductive metal oxides
such as titanium oxide; or conductive polymers such as
polyphenylene derivatives, and these may be used either alone or as
a mixture of two or more types. The conductor may be included in 1%
by weight to 30% by weight with respect to the total weight of the
positive electrode active material layer.
In addition, the binder performs a role of enhancing adhesion
between the positive electrode active material particles and
adhesive strength between the positive electrode active material
and the current collector. Specific examples thereof may include
polyvinylidene fluoride (PVDF), a vinylidene
fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl
alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch,
hydroxypropyl cellulose, regenerated cellulose, polyvinyl
pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an
ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM,
styrene-butadiene rubber (SBR), fluorine rubber, or various
copolymers thereof. These may be used either alone or as a mixture
of two or more types. The binder may be included in 1% by weight to
30% by weight with respect to the total weight of the positive
electrode active material layer.
In addition, solvents generally used in the art may be used as the
solvent for dissolving or dispersing the conductor and the binder
as well as the positive electrode active material when preparing
the composition for forming a positive electrode active material
layer, and specific examples thereof may include dimethyl sulfoxide
(DMSO), isopropyl alcohol, N-methyl-pyrrolidone (NMP), acetone,
water or the like. These may be used either alone or as a mixture
of two or more types. The amount of the solvent used is sufficient
when the amount is capable of dissolving or dispersing the positive
electrode active material, the conductor and the binder considering
slurry coating thickness and preparation yield, and the slurry has
viscosity to exhibit excellent thickness uniformity when coated for
preparing the positive electrode thereafter.
Meanwhile, the positive electrode current collector is not
particularly limited as long as it has high conductivity without
inducing chemical changes in the battery, and examples thereof may
include stainless steel, aluminum, nickel, titanium, baked carbon,
or aluminum or stainless steel of which surface is treated with
carbon, nickel, titanium, silver or the like. In addition, the
positive electrode current collector may have a thickness of 3
.mu.m to 500 .mu.m, and adhesive strength of the positive electrode
active material may be enhanced by forming fine unevenness on the
surface of the current collector. For example, various forms such
as films, sheets, foil, nets, porous bodies, foams and non-woven
fabrics may be used.
In addition, as another method, the positive electrode may be
prepared by casting the composition for forming a positive
electrode active material layer on a separate support, and then
laminating a film obtained from being peeled off from this support
on the positive electrode current collector. Herein, the support
may be used with particular limit as long as it has a releasing
property so as to readily peel off the positive electrode active
material layer such as a releasing polymer film.
Next, Step 2 in the method for testing a cycle life of a positive
electrode active material for a secondary battery according to one
embodiment of the present disclosure is a step of preparing a
half-cell using the positive electrode prepared in Step 1 and a
lithium negative electrode.
The half-cell may be prepared using common methods except that the
positive electrode prepared in Step 1 is used, and a lithium
negative electrode is used as a counter electrode for the positive
electrode. In addition, the shape of the half-cell is not
particularly limited, and specifically, a coin half-cell may be
used.
Specifically, the separator in the preparation of the half-cell
separates the negative electrode and the positive electrode and
provides a lithium ion migration path, and is not particularly
limited as long as it is used as a separator in common lithium
secondary batteries, and particularly, separators having an
excellent liquid electrolyte moisture permeating ability while
having low resistance for electrolyte ion migration are preferred.
Specifically, porous polymer films, for example, porous polymer
films prepared with polyolefin-based polymers such as ethylene
homopolymers, propylene homopolymers, ethylene/butene copolymers,
ethylene/hexene copolymers and ethylene/methacrylate copolymers, or
laminated structures of two or more layers thereof, may be used. In
addition, common porous non-woven fabrics, for example, non-woven
fabrics made of high melting point glass fiber, polyethylene
terephthalate fiber or the like may be used. In addition, coated
separators including ceramic components or polymer materials may
also be used for securing heat resistance or mechanical strength,
and selectively, the separator may be used in single layer or
multilayer structures.
As the electrolyte, organic-based liquid electrolytes,
inorganic-based liquid electrolytes, solid polymer electrolytes,
gel-type polymer electrolytes, solid inorganic electrolytes,
molten-type inorganic electrolytes and the like capable of being
used in manufacturing lithium secondary batteries may be used,
however, the electrolyte is not limited thereto.
Specifically, the electrolyte may include an organic solvent and a
lithium salt.
The organic solvent may be used without particular limit as long as
it is capable of performing a role of a medium in which ions
participating in a battery electrochemical reaction are capable of
migrating. Specific examples of the organic solvent may include
ester-based solvents such as methyl acetate, ethyl acetate,
.gamma.-butyrolactone or .epsilon.-caprolactone; ether-based
solvents such as dibutyl ether or tetrahydrofuran; ketone-based
solvents such as cyclohexanone; aromatic hydrocarbon-based solvents
such as benzene and fluorobenzene; carbonate-based solvents such as
dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl
carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate
(EC) or propylene carbonate (PC); alcohol-based solvents such as
ethyl alcohol and isopropyl alcohol; nitriles such as R--CN (R is a
C2 to C20 linear, branched or cyclic hydrocarbon group, and may
include double bond aromatic rings or ether bonds); amides such as
dimethylformamide; dioxolanes such as 1,3-dioxolane; sulfolanes, or
the like. Among these, the electrolyte may include a
carbonate-based solvent, and more specifically, may include a
mixture of cyclic carbonate (for example, ethylene carbonate,
propylene carbonate or the like) and linear carbonate-based
compounds (for example, ethylmethyl carbonate, dimethyl carbonate,
diethyl carbonate or the like), and even more specifically, may
include a mixture mixing cyclic carbonate and chain carbonate in a
volume ratio of 1:1 to 1:9.
In addition, the lithium salt may be used without particular limit
as long as it is a compound capable of providing lithium ions used
in lithium secondary batteries. Specific examples of the lithium
salt may include LiPF.sub.6, LiClO.sub.4, LiAsF.sub.6, LiBF.sub.4,
LiSbF.sub.6, LiAlO.sub.4, LiAlCl.sub.4, LiCF.sub.3SO.sub.3,
LiC.sub.4F.sub.9SO.sub.3, LiN(C.sub.2F.sub.5SO.sub.3).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LlN(CF.sub.3SO.sub.2).sub.2,
LiCl, LiI, LiB(C.sub.2O.sub.4).sub.2 or the like. The lithium salt
is favorably used in a concentration range of 0.1 M to 2.0 M. When
the lithium salt concentration is in the above-mentioned range, the
electrolyte has proper conductivity and viscosity, and therefore,
may exhibit excellent electrolyte properties, and lithium ions may
effectively migrate.
Next, Step 3 in the method for testing a cycle life of a positive
electrode active material for a secondary battery according to one
embodiment of the present disclosure is a step of measuring
discharge capacity at each cycle for the half-cell prepared in Step
2.
In the measuring of discharge capacity, charge and discharge are
carried out under a high temperature and high pressure condition
more severe than in common operations of secondary batteries so
that the positive electrode active material is quickly degenerated
in a range that does not damage the positive electrode active
material and the half-cell including the same.
Specifically, the measuring of discharge capacity may be carried
out by repeating 5 cycles to 10 cycles with a charge and discharge
process of charge-maintain-discharge including processes of
charging the half-cell to 4.4 V to 4.5 V under a constant current
condition; maintaining for 5 hours to 10 hours in a voltage range
of 4.4 V to 4.5 V at 50.degree. C. to 80.degree. C.; and
discharging to 3.0 V under a constant voltage condition as 1 cycle,
and measuring discharge capacity at each cycle.
More specifically, the charge process may be carried out under a
constant current condition of 0.2 C to 0.5 C. Even more
specifically, the charge process may be carried out under a
constant current condition of 0.2 C.
The maintaining process after the charging is for accelerating
degeneration of the positive electrode material under a severe
condition. When the maintaining process is not carried out, the
rate of degeneration is low, and an effect of shortening the time
of testing a cycle life property is difficult to obtain.
More specifically, the maintaining process may be carried out for 8
hours to 10 hours in a voltage range of 4.45 V to 4.5 V at
60.degree. C. to 70.degree. C., and even more specifically, may be
carried out for 10 hours at a voltage of 4.45 V at 70.degree.
C.
The number of times of the charge and discharge process including
processes of charge-maintain-discharge as above may be properly
determined depending on the types of the positive electrode active
material subject to test. Specifically, 5 cycles to 10 cycles may
be carried out. More specifically, 5 cycles to 8 cycles may be
carried out when considering concern for degeneration of the
positive electrode active material caused by driving under high
temperature and high voltage.
Even more specifically, the measuring of discharge capacity may be
carried out by repeating 5 cycles to 8 cycles with a charge and
discharge process of charge-maintain-discharge including processes
of charging the half-cell to 4.4 V to 4.5 V under a constant
current condition of 0.2 C; maintaining for 10 hours at a voltage
of 4.45 V at 70.degree. C.; and discharging to 3.0 V under a
constant voltage condition as 1 cycle, and measuring discharge
capacity at each cycle.
Next, Step 4 in the method for testing a cycle life of a positive
electrode active material for a secondary battery according to one
embodiment of the present disclosure is a step of calculating a
discharge capacity retention rate at each cycle with respect to
discharge capacity at the first cycle using the measured discharge
capacity.
Discharge capacity retention rate calculation at each cycle may be
carried out using common discharge capacity retention rate
calculation methods.
As described above, the method for testing a cycle life of a
positive electrode active material according to one embodiment of
the present disclosure may predict and assess a life cycle of a
positive electrode active material with high accuracy and excellent
reliability in a short period of time by accelerating degeneration
of the positive electrode active material using a method of
maintaining for a certain period of time at high temperature and
high voltage. Accordingly, cycle life properties of a cell may be
readily predicted in a step of testing a positive electrode active
material without manufacturing and directly testing the cell, which
enables efficient choice of positive electrode active materials,
and is also economically advantageous since cell manufacturing for
actual tests is reduced.
Hereinafter, examples of the present disclosure will be described
in detail so as for those skilled in the art to readily carry out
the present disclosure. However, the present disclosure may be
implemented into various different forms, and is not limited to the
examples described herein.
Preparation Example 1-1 to Preparation Example 1-3: Preparation of
Positive Electrode
A positive electrode active material, a conductor and a binder
listed in Table 1 subject to a cycle life property test were mixed
in a weight ratio of 90:5:5. The result was mixed with
N-methylpyrrolidone, a solvent, to prepare a composition for
forming a positive electrode (viscosity: 5,000 mPas). The
composition for forming a positive electrode was coated on an
aluminum current collector, and the result was dried at 130.degree.
C. and rolled to prepare a positive electrode.
TABLE-US-00001 TABLE 1 Positive Electrode Active Material Lithium
Composite Doping Element Category Metal Oxide Mg (ppm) Al (ppm)
Conductor Binder Preparation LiCoO.sub.2 900 500 Carbon PVdF
Example 1-1 Black Preparation LiCoO.sub.2 -- 500 Carbon PVdF
Example 1-2 Black Preparation LiCoO.sub.2 -- 300 Carbon PVdF
Example 1-3 Black
Preparation Example 2-1 to Preparation Example 2-3: Preparation of
Half-Cell
An electrode assembly was prepared by providing a separator between
a positive electrode and a lithium negative electrode listed in the
following Table 2. The electrode assembly was placed inside a case,
and a liquid electrolyte was injected to the inside of the case to
prepare a half-cell. Herein, the liquid electrolyte was prepared by
dissolving lithium hexafluorophosphate (LiPF.sub.6) having a
concentration of 1.0 M in an organic solvent formed with ethylene
carbonate/dimethyl carbonate/ethyl methyl carbonate (mixed volume
ratio of EC/DMC/EMC=3/4/3).
TABLE-US-00002 TABLE 2 Positive Negative Category Electrode
Electrode Separator Preparation Preparation Li Thin Film Porous
Example 2-1 Example 1-1 Polyethylene Preparation Preparation
Example 2-2 Example 1-2 Preparation Preparation Example 2-3 Example
1-3
Comparative Preparation Example 2-1 to Comparative Preparation
Example 2-3: Manufacture of Lithium Secondary Battery
A negative electrode active material, carbon black and a binder
listed in the following Table 3 were mixed in a weight ratio of
85:10:5. The result was mixed with an N-methylpyrrolidone solvent
to prepare a composition for forming a negative electrode. The
composition for forming a negative electrode was coated on a copper
current collector to prepare a negative electrode.
An electrode assembly was prepared by providing a separator between
a positive electrode and a negative electrode listed in the
following Table 3. The electrode assembly was placed inside a
square battery case, and a liquid electrolyte was injected to the
inside of the case to prepare a lithium secondary battery. Herein,
the liquid electrolyte was prepared by dissolving lithium
hexafluorophosphate (LiPF.sub.6) having a concentration of 1.15 M
in an organic solvent formed with ethylene carbonate/dimethyl
carbonate/ethyl methyl carbonate (mixed volume ratio of
EC/DMC/EMC=3/4/3).
TABLE-US-00003 TABLE 3 Negative Electrode Negative Electrode
Positive Active Con- Category Electrode Material ductor Binder
Separator Comparative Preparation MCMB Carbon PVdF Porous
Preparation Example 1-1 (Mesocarbon Black Poly- Example 2-1
Microbead) ethylene Comparative Preparation Preparation Example 1-2
Example 2-2 Comparative Preparation Preparation Example 1-3 Example
2-3
Example 1 to Example 3: Evaluation on Half-Cell Properties
The half-cells of Preparation Example 2-1 to Preparation Example
2-3 were charged to 4.45V under a constant current condition of 0.2
C. After that, the half-cells were maintained for 10 hours at a
voltage of 4.45 V at 70.degree. C. and then discharged to 3.0 V
under a constant voltage condition. With a charge and discharge
process including the above-mentioned charge and discharge as 1
cycle, 6 cycles were repeated, and discharge capacity at each cycle
was measured. A discharge capacity retention rate at each cycle
with respect to discharge capacity at the first cycle using the
measured discharge capacity is calculated. The results are shown in
FIG. 1.
Comparative Example 1 to Comparative Example 3: Evaluation on
Lithium Secondary Battery Properties
The lithium secondary batteries of Comparative Preparation Example
2-1 to Comparative Preparation Example 2-3 were charged to 4.4 V
under a constant current condition of 0.8 C, and then maintained
for 24 hours at a voltage of 4.4 V at 40.degree. C. and discharged
to 3.0 V under a constant voltage condition. With a charge and
discharge process including the above-mentioned charge and
discharge as 1 cycle, 120 cycles were repeated, and discharge
capacity at each cycle was measured. A discharge capacity retention
rate at each cycle with respect to discharge capacity at the first
cycle using the measured discharge capacity is calculated. The
results are shown in FIG. 2.
When referring to FIG. 1 to FIG. 3, cycle life properties with
similar trends were able to be identified in a significantly
shorter period of time when testing a cycle life of the positive
electrode active material using the methods according to Example 1
to Example 3. From such results, it can be seen that positive
electrode active materials having excellent cycle life properties
are capable of being readily predicted prior to manufacturing
actual cells when carrying out tests using the methods according to
the present disclosure.
* * * * *